1. Field of the Invention
The present invention relates to a sewing machine, and more particularly to a sewing machine with means including sensors and electronic controls for producing seams which terminate at a predetermined point or location.
2. Description of Related Art
The following methods have been used for a long time in the sewing industry for locating the last needle insertion of a seam at a predetermined place, such place being determined as a desired given distance from the edge of the material being sewn:
a) After visual detection of the edge, the sewing machine may be stopped at a suitable moment, the sewing needle being in raised position. Thereupon the presser foot is brought into its raised position. The piece being sewn in then moved by hand in such a manner that the following needle insertion strikes the predetermined place.
b) After visual detection of the edge, the sewing machine may be stopped at a suitable time at which the sewing needle is in raised position while the presser foot remains down. The stitch regulator is now actuated by hand in such a manner that the sewing needle points to the predetermined place. The last stitch is then performed.
Both methods are time consuming since the method steps to be performed must be carried out by the eye, the brain and the hands of the operator. A more rational manner of operation would be desirable, in which the method steps are carried out automatically by the sewing machine.
Such a sewing machine is known from West German Pat. No. 31 50 141. In that example, the sewing needle is stopped at a predetermined point in the material being sewn, either at the end of the seam, or at a point which is characterized by an interruption of the seam or by a change in direction of the seam. For this purpose the known sewing machine is equipped essentially with a sensor which is arrange din front of the sewing needle and scans the edge of the material being sewn. An electronic control executes program instructions for the exact positioning of the last needle insertion. A potentiometer is fastened at the end of the setting shaft, from the angular position of which dependent values can be obtained which represent the length of stitch set. A pulse transmitter acts during the advancing phase of the feed dog and in cooperation with the aforementioned potentiometer, to give information as to the present state of advance of the stitch which has just been produced. An electromagnetically actuatable multi-member parallel-crank transmission permits a temporary lowering of the feed dog into a position which is inactive for the transport of the material being sewn.
This known sewing machine has the following disadvantages:
1. In order to be able to influence the length of the last stitch, two pick-ups are necessary, namely a potentiometer which represents the stitch length set in measurement values and a rotary pulse transmitter provided in the lower shaft and which breaks down the length of stitch set into partial lengths, whereby the latter become countable.
2. Since various levers are arranged behind the potentiometer and have unavoidable play and elasticity, the transport path moved over by the feed dog is dependent on the speed of sewing and is not necessarily proportional to the measurement value of the potentiometer.
3. The transport path of the feed dog which has been moved over is related to the angle of rotation of the associated rotary pulse transmitter by a trigonometric function. If one assumes that the graduations on the disk of the rotary pulse transmitter are, as is customary, a uniform distance apart, then measurement errors occur due to the fact that at the start and at the end of each forward step smaller transport paths per angle of rotation are moved over, and in the central part of the forward step longer transport paths per angle of rotation are moved over. The measurement errors resulting from this non-linear relationship must be accepted in the known sewing machine.
4. The parts necessary for the sudden lowering of the feed dog increase the structural cost, and when they move cause forces of inertia which can have an unfavorable effect on the kinematics of the feed dog.
5. The piece being sewn which is held between the smooth top side of the throat plate and the smooth bottom side of the presser foot when the feed dog is lowered is not held in stable position when pulling forces are exerted on the piece being sewn.